1. Field of the Invention
The present invention relates to a projector and more precisely it relates to an apparatus for enlarging and projecting an image of an object, such as liquid crystal panel, Braun tube, original picture film, or a testpiece.
2. Description of Related Art
In recent years, projectors which project an enlarged image of an object to be indicated (object to be projected), such as a liquid crystal indication panel, Braun tube (CRT), or an original picture film have been developed.
FIG. 13 shows a known projector of the kind, in which a liquid crystal panel is used as an object to be indicated. In FIG. 13, numeral 1 designates a projector body which is provided on its front face with an indication window with a transmission type screen 2 as a projection plane. A transmission type liquid crystal display panel 3 is provided on the rear portion of the projector body 1. In front of the transmission type liquid crystal display panel 3 is located a O projection lens 4 which has an optical axis O perpendicular to the plane of the projection screen 2. The liquid crystal display panel 3 has a center on the optical axis O of the projection lens 4 and lies in a plane parallel with the plane of the projection screen 2. Numeral 5 designates a light source for illuminating the liquid crystal display panel 3 from behind.
Light emitted from the light source 5 is transmitted through the liquid crystal display panel 3, so that an indication image of the liquid crystal display panel 3 is enlarged by the projection lens 4 to project the same onto the projection screen 2. The image projected on the projection screen can be viewed from the front side of the apparatus. In the arrangement shown in FIG. 13, the transmission type screen 2 as a projection plane is provided on the front face of the projector body 1, and both the liquid crystal display panel 3 and the projection lens 4 are provided in the projector body 1. Alternatively, it is also known to provide an external screen outside the projector body 1, so that an image is projected on the external screen.
In the projector as mentioned above, an increase of an optical path length from the projection lens 4 to the screen 2 makes it possible to spread the luminous flux transmitted through the projection lens 4 at a larger angle, and accordingly it is desirable to space the projection lens 4 from the screen 2 at a longer distance in order to produce a larger image on the screen 2.
However, since the projection lens 4 and the liquid crystal display panel 3 are aligned on the optical axis perpendicular to the plane of the screen 2 in the known apparatus, as shown in FIG. 13, accordingly, an increased distance between the projection lens 4 and the screen 2 inevitably leads to an increased size of the apparatus as a whole.
It is also known to incline the projection lens 4 with respect to the projection screen 2, so that the image is projected on the screen 2 from the inclined direction, as shown in FIG. 14. This arrangement of the inclined projection lens 4 makes it possible to locate the projection lens 4 closer to the screen 2 while guaranteeing an increased optical path length therebetween. In the arrangement, it is also possible to reduce the size (depth) of a projector which has the transmission type screen 2 at the front face thereof and the liquid crystal display panel 3 and the projection lens 4 in the projector body 1. In the arrangement shown in FIG. 14, the liquid crystal display panel 3 is inclined at a predetermined inclination angle .theta. with respect to a plane perpendicular to the optical axis O of the projection lens 4. An extension plane 3a of the display surface of the liquid crystal display panel 3 intersects an extension plane of the principal plane 4a of the projection lens 4 at an intersecting line P at which the extension plane of the principal plane 4a intersects an extension plane 2a of the screen 2, so that an indication image of the liquid crystal display panel 3 can be formed on the screen 2 in the state of "in focus". This is called "Scheimpflug's law".
However, in the arrangement shown in FIG. 14, the image projected on the screen 2 is elliptically distorted or deformed. This is because there is a difference in magnification between the points of the image projected on the screen 2.
The elliptical distortion of an image will be discussed below in more detail.
FIG. 15 which shows an optical system of a known projection type indicating apparatus. Ao designates the indication image of the liquid crystal display panel 3, A the projection image projected on the screen 2, F and F' the focal points of the projection lens 4, .theta. the intersecting angle of the extension plane 3a of the liquid crystal display panel 3 and the extension plane 4a of the principal plane of the projection lens 4, .theta.' the intersecting angle of the extension plane 4a of the principal plane of the projection lens 4 and the extension plane of the screen 2, respectively. Note that .theta.&lt;0 and .theta.'&gt;0. The origin of the coordinate x-y-z is identical to the focal point F and the origin of the coordinate x'-y'-z' is identical to the focal point F'. In FIG. 15, supposing that the z coordinate of a point Zo on the optical axis O of the indication image Ao is represented by zo, the z' coordinate of a point Zo' on the optical axis O of the projection image A is represented by zo', and the focal length of the projection lens 4 is f, if the following equation is given, the extension plane 3a of the liquid crystal display panel 3 and the extension plane 4a of the principal plane of the projection lens 4 intersect at the point P, so that the projection image A projected on the screen 2 is in focus: EQU f/zo=-zo'/f=tan .theta.'/tan .theta.
Here, since z=zo+y.multidot.tan .theta. and z'=zo'+y'.multidot.tan .theta.', the magnification m of the points of the projection image A with respect to the indication image Ao in the directions x, y and z is given by the following equations: ##EQU1## and the magnification M in the y direction is: ##EQU2##
Consequently, in the case of a projection type display apparatus as shown in FIG. 14 in which an image is projected from a downwardly inclined direction onto the projection screen 2, even if the display image Ao of the liquid crystal display panel 3 is rectangular, as shown in FIG. 16a, the projected image A formed on the screen 2 is deformed into an inverted trapezoid, as shown in FIG. 16b. The same is true in a projector in which a Braun tube or an original picture film or the like is used as an indicator.
The above discussion can be applied to an inspecting projector in which a testpiece (object to be projected) is illuminated with an illumination light to enlarge and project an image thereof.
FIG. 17 shows a conventional inspecting projector in which the liquid crystal display panel 3 in FIGS. 14 and 15 is replaced with a testpiece 3'. In FIG. 17, a light source 8a for a bright field inspection and a light source 8b for a dark field inspection are provided to illuminate the testpiece 3'. A mirror 7 is located in front of the screen 2. Other construction of the arrangement shown in FIG. 17 is optically identical to that of FIGS. 14 and 15. The bright field inspecting light source 8a and the dark field inspecting light source 8b are selectively made ON in accordance with the state of the testpiece 3'. When the bright field inspecting light source 8a is made ON, an inspector can view a bright image of the testpiece surface formed on the screen 2 and a dark flaw or foreign matter (dust etc.). On the other hand, when the dark field inspecting light source 8b is made ON, an inspector can view a bright flaw or foreign matter and a dark image of the testpiece surface formed on the screen 2.
In the inspecting projector as mentioned above, even if the testpiece 3' is rectangular, as shown in FIG. 16a, the projected image A formed on the screen 2 is deformed into an inverted trapezoid, as shown in FIG. 16b, thus resulting in a difficulty in viewing and a decreased precision of inspection.